Mass spectrometers work by ionizing molecules and then sorting and identifying the molecules based on their mass-to-charge (m/z) ratios. Two key components in this process include the ion source, which generates ions, and the mass analyzer, which sorts the ions. Several different types of ion sources are available for mass spectrometers. Each ion source has particular advantages and is suitable for use with different classes of compounds. Different types of mass analyzers are also used. Each has advantages and disadvantages depending upon the type of information needed.
Much of the advancement in liquid chromatography/mass spectrometry (LC/MS) over the last ten years has been in the development of new ion sources and techniques that ionize analyte molecules and separate the resulting ions from the mobile phase. Earlier LC/MS systems performed at sub-atmospheric pressures or under partial vacuum, whereas API occurs at atmospheric pressure. In addition, historically in these older systems all components were generally under vacuum, whereas API occurs external to the vacuum and the ions are then transported into the vacuum.
Previous approaches were successful only for a very limited number of compounds.
The introduction of API techniques greatly expanded the number of compounds that can be successfully analyzed using LC/MS. In this technique, analyte molecules are first ionized at atmospheric pressure. The analyte ions are then spatially and electrostatically separated from neutral molecules. Common API techniques include: electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI). Each of these techniques has particular advantages and disadvantages.
Electrospray ionization is the oldest technique and relies in part on chemistry to generate analyte ions in solution before the analyte reaches the mass spectrometer. The LC eluent is sprayed (nebulized) into a chamber at atmospheric pressure in the presence of a strong electrostatic field and heated drying gas. The electrostatic field charges the LC eluent and the analyte molecules. The heated drying gas causes the solvent in the droplets to evaporate. As the droplets shrink, the charge concentration in the droplets increases. Eventually, the repulsive force between ions with like charges exceeds the cohesive forces and the ions are ejected (desorbed) into the gas phase. The ions are attracted to and pass through a capillary or sampling orifice into the mass analyzer. Some gas-phase reactions, mostly proton transfer and charge exchange, can also occur between the time ions are ejected from the droplets and the time they reach the mass analyzer.
Electrospray is particularly useful for analyzing large biomolecules such as proteins, oligonucleotides, peptides etc. The technique can also be useful for analyzing polar smaller molecules such as benzodiazepines and sulfated conjugates. Other compounds that can be effectively analyzed include ionizing salts and organic dyes.
Large molecules often acquire more than one charge. Multiple charging provides the advantage of allowing analysis of molecules as large as 150,000 u even though the mass range (or more accurately mass-to-charge range) for a typical LC/MS instrument is around 3000 m/z. When a large molecule acquires many charges, a mathematical process called deconvolution may be used to determine the actual molecular weight of the analyte.
A second common technique performed at atmospheric pressure is atmospheric pressure chemical ionization (APCI). In APCI, the LC eluent is sprayed through a heated vaporizer (typically 250-400° C.) at atmospheric pressure. The heat vaporizes the liquid and the resulting gas phase solvent molecules are ionized by electrons created in a corona discharge. The solvent ions then transfer the charge to the analyte molecules through chemical reactions (chemical ionization). The analyte ions pass through a capillary or sampling orifice into the mass analyzer. APCI has a number of important advantages. The technique is applicable to a wide range of polar and nonpolar molecules. The technique rarely results in multiple charging like electrospray and is, therefore, particularly effective for use with molecules of less than 1500 u. For these reasons and the requirement of high temperatures, APCI is a less useful technique than electrospray in regards to large biomolecules that may be thermally unstable. APCI is used with normal-phase chromatography more often than electrospray is because the analytes are usually nonpolar.
Atmospheric pressure photoionization for LC/MS is a relatively new technique. As in APCI, a vaporizer converts the LC eluent to the gas phase. A discharge lamp generates photons in a narrow range of ionization energies. The range of energies is carefully chosen to ionize as many analyte molecules as possible while minimizing the ionization of solvent molecules. The resulting ions pass through a capillary or sampling orifice into the mass analyzer. APPI is applicable to many of the same compounds that are typically analyzed by APCI. It shows particular promise in two applications, highly nonpolar compounds and low flow rates (<100 ul/min), where APCI sensitivity is sometimes reduced. In all cases, the nature of the analyte(s) and the separation conditions have a strong influence on which ionization technique: electrospray, APCI, or APPI will generate the best results. The most effective technique is not always easy to predict.
Each of these techniques described above ionizes molecules through a different mechanism. Unfortunately, none of these techniques are universal sample ion generators. While many times the lack of universal ionization could be seen as a potential advantage, it presents a serious disadvantage to the analyst responsible for rapid analysis of samples that are widely divergent. An analyst faced with very limited time and a broad array of numerous samples to analyze is interested in an ion source capable of ionizing as many kinds of samples as possible with a single technique and set of conditions. Unfortunately, such an API ion source technique has not been available.
Attempts have been made to improve sample ionization coverage by the use of rapid switching between positive and negative ion detection. Rapid positive/negative polarity switching does result in an increase in the percentage of compounds detected by any API technique. However, it does not eliminate the need for more universal API ion generation.
For these reasons it would be desirable to employ a source that can provide the benefits of multiple sources (electrospray, APCI, and APPI) combined, but not have the individual limitations. In addition, it would be desirable to have a source which does not require switching from one source to another source or which requires manual operations to engage the source. Thus, there is a need to provide a multimode ion source that can ionize a variety of samples quickly, efficiently and effectively.
To best accommodate two or more different ionization sources in a single ion source apparatus, it is advantageous to avoid having one ionization source mechanism interfere with the other ionization source mechanism(s). One concern that may arise when an ESI source is used in conjunction with another ionization source is ensuring effective drying of the aerosol containing the analyte ions. Since ESI sources normally do not use a vaporizer tube because of the possibility of ion discharge to walls of the tube, it is particularly advantageous to provide an alternative technique for drying the aerosol that does not interfere with either the operation of the other ionization source or the flow of analyte ions toward the entrance of the mass spectrometer.
In multimode sources that include both an ESI source and an APCI source (ESI/APCI), it is important that the downstream flow of ions generated by the ESI source not substantially interfere with either the corona discharge produced by the APCI corona needle or the ions generated by the corona discharge. Such interference can reduce the ion-generation efficiency of the APCI source and can also reduce the number of APCI-generated ions that reach the entrance of the mass spectrometer. In addition, the voltage levels maintained at various portions of the multimode ion source apparatus used to guide ions downstream and toward the entrance of the mass spectrometer can influence the electric field at the corona needle and thereby cause the corona discharge current to vary, resulting in inconsistent operation of the APCI source.